Disorder and compositional variation in the lillianite homologous series.

Abstract

<jats:title>Abstract</jats:title><jats:p>High resolution transmission electron microscopy studies on the lillianite group minerals from the Ivigtut cryolite deposit, Ivigtut, South Greenland revealed the existence of disordered intergrowths of lillianite/gustavite-like blocks (<jats:italic>N</jats:italic>− 4) and heyrovskyite-like (<jats:italic>N</jats:italic>= 7) structural blocks. One disorder sequence is examined in detail, which gave an average homologue number N = 4.92 corresponding to a composition of Pb<jats:sub>3.92</jats:sub><jats:sub>2<jats:italic>x</jats:italic></jats:sub>Bi<jats:sub>2·<jats:italic>x</jats:italic></jats:sub>Ag<jats:sub><jats:italic>x</jats:italic></jats:sub>S<jats:sub>6.92</jats:sub>with x ≈ 1.2. An Axial Next-Nearest Neighour Ising model was used to follow the fluctuations in the average homologue number<jats:italic>N</jats:italic>across the crystal. This yielded compositional fluctuations of the order of 70–170 Å over a 1800 Å region of the crystal, with a 202 Å lamella of ordered vikingite. Trends in the randomness of the gustavite-vikingite intergrowth were evaluated and the dominant slab sequence was found to be 4,4,4,7 and 4,4,7,7, suggesting that some longer period homologues may be stable. A number of defects were noted in which changes in slab widths were accommodated. The origin of these partially ordered/disordered lillianite homologues is discussed.</jats:p>